Coated articles are known in the art for use in window applications such as insulating glass (IG) window units and/or the like. It is known that in certain instances, it is desirable to heat treat (e.g., thermally temper, heat bend and/or heat strengthen) such coated articles for purposes of tempering, bending, or the like. Heat treatment (HT) of coated articles typically requires use of temperature(s) of at least 580 degrees C., more preferably of at least about 600 degrees C. and still more preferably of at least 620 degrees C. Such high temperatures (e.g., for 5-10 minutes or more) often cause coatings to break down and/or deteriorate or change in an unpredictable manner. Thus, it is desirable for coatings to be able to withstand such heat treatments (e.g., thermal tempering), if HT is desired, in a predictable manner that does not significantly damage the coating.
To be able to produce tempered coated glass articles, architectural coatings such as low-E coatings typically need to be heat treated. As tempered glass is more expensive compared to non-tempered glass, tempered coated articles are typically only utilized if required. Thus, it would be desirable for two products to be offered in the market—one heat treated and one that is not heat treated—namely the actual “as coated” (AC) product with a specific color and thermal performance, as well as a heat treated (HT) mate product which substantially matches the AC product regarding performance and color following heat treatment such as thermal tempering. It is desirable if the color match between AC and HT mates are close enough such that the two products are virtually or essentially indistinguishable to the naked eye when applied side by side in a particular manner. This is achieved when (a) the AC and HT products have the same or similar coating, (b) the coated article can be heat treated (e.g., thermally tempered), and (c) the heat treated coated article has a low ΔE* value (e.g., a ΔE* value of no greater than 5.0, more preferably no greater than 4.0). The low ΔE* value, due to HT, indicates for example that the color of the coated article does not significantly change due to the HT so that the HT version substantially matches the non-HT version of the coated article.
The term ΔE* (and ΔE) is well understood in the art and is reported, along with various techniques for determining it, in ASTM 2244-93 as well as being reported in Hunter et. al., The Measurement of Appearance, 2nd Ed. Cptr. 9, page 162 et seq. [John Wiley & Sons, 1987]. As used in the art, ΔE* (and GE) is a way of adequately expressing the change (or lack thereof) in reflectance and/or transmittance (and thus color appearance, as well) in an article after or due to heat treatment. ΔE may be calculated by the “ab” technique, or by the Hunter technique (designated by employing a subscript “H”). ΔE corresponds to the Hunter Lab L, a, b scale (or Lh, ah, bh). Similarly, ΔE* corresponds to the CIE LAB Scale L*, a*, b*. Both are deemed useful, and equivalent for the purposes of this invention. For example, as reported in Hunter et. al. referenced above, the rectangular coordinate/scale technique (CIE LAB 1976) known as the L*, a*, b* scale may be used, wherein: L* is (CIE 1976) lightness units; a* is (CIE 1976) red-green units; b* is (CIE 1976) yellow-blue units; and the distance ΔE* between L*o a*o b*o and L*1 a*1 b*1 is: ΔE*=[(ΔL*)2+(Δa*)2+(Δb*)2]1/2, where: ΔL*=L*1−L*o; Δa*=a*1−a*o; Δb*=b*1−b*o; where the subscript “o” represents the coating (coated article) before heat treatment and the subscript “1” represents the coating (coated article) after heat treatment; and the numbers employed (e.g., a*, b*, L*) are those calculated by the aforesaid (CIE LAB 1976) L*, a*, b* coordinate technique. When, for example, glass side reflective ΔE* values are measured, then glass side reflective a*, b* and L* values are used. In a similar manner, ΔE may be calculated using the above equation for ΔE*, i.e., ΔE*=[(ΔL*)2+(Δa*)2+(Δb*)2]1/2, by replacing a*, b*, L* with Hunter Lab values ah, bh, Lh. Also within the scope of this invention and the quantification of ΔE* are the equivalent numbers if converted to those calculated by any other technique employing the same concept of ΔE* as defined above. ΔE* is also defined in U.S. Pat. No. 7,964,284, which is incorporated herein by reference.
During the ITT process, the coated glass may be heated to temperature(s) between 650° C. and 750° C. and then subsequently cooled down at a high rate to create intrinsic stress, which results in a higher strength and, as soon as breakage occurs, into a fine breaking pattern. This high temperature treatment causes different processes within the coating (e.g. oxidation, re- crystallization, diffusion, volume changes, stress increase or relaxation etc.) which tend to alter color values of the coated article. Thus, it is desirable that changes (e.g., color changes indicated by changes in a* and/or b* color values) in the coating, which are caused by the HT process, remain predictable with varying times of HT.
As will be explained below, it would be desirable if the AC and HT mates substantially match (i.e., substantially matching the AC product which is not heat treated, and the HT product after heat treatment) with respect to color even though the HT version may be heat treated for different periods of time within reason. Practically speaking, a low-E coating is applied to various different glass thicknesses between 4 mm and 12 mm and each of those glass thicknesses requires different heating regimes during the HT process to achieve the required tempered glass properties. In general, thicker glass needs to be heated for a longer time and/or at higher temperatures and it is cooled at lower rates. And coating products are typically sold to various customers who run different models and types of furnaces, e.g. irradiation furnaces, convection furnaces or hybrid models. In each of these furnace types, the heat transfer into glass and coating differs.
Accordingly, it would be desirable to achieve a thermally stable product allowing a HT product to substantially match annealed and non-tempered products with similar or same coatings with respect to color, after the HT process for the HT product independent of the glass thickness and the different furnace types within reason. In other words, it would be desirable for the HT product realize a low ΔE* value, such as a ΔE* value of no greater than 5.0, more preferably no greater than 4.0, within certain HT time periods such as one or more of 10 minutes, 16 minutes, and/or 24 minutes.
Due to the mentioned processes occurring within the coating during the HT process, some performance and color change cannot be avoided. However, it would be desirable for most or much of these changes to occur at the beginning of, or within a short period of, a HT process (e.g., within the first 8-16 minutes, or within the first 10-12 or 10-16 minutes of HT), so that the heat treated product substantially hits the final desired color values within the first 16 minutes or so of the HT process, so that the product can remain substantially stable with respect to color change over an increased tempering time period of the HT process, independent of the furnace type, if that should occur. Of course, sometimes the HT period will be less than 16 minutes. For example, assuming an example HT process of 24 minutes, it would be desirable for the coated article to substantially realize the final desired color values within the first 16 minutes or so of the HT process, so that the product can remain substantially stable with respect to color change over the time period from about 16 to 24 minutes. Stated another way, it would be desirable for the coated article to realize a lower ΔE* value for the 16-24 minutes period of HT than for the 0-16 minutes period of the twenty-four minute heat treatment process. Therefore, for example, a pair of thermally tempered products with the same coating would substantially match when one was heat treated for 12 minutes and the other for 20 minutes.
In certain situations, designers of coated articles strive for a combination of desirable visible transmission, desirable color, low emissivity (or emittance), and low sheet resistance (Rs). Low-emissivity (low-E) and low sheet resistance characteristics permit such coated articles to block significant amounts of IR radiation so as to reduce for example undesirable heating of vehicle or building interiors.
Example embodiments of this invention relate to a coated article including a low emissivity (low-E) coating supported by a glass substrate. The coated article may be heat treated (e.g., thermally tempered, heat bent and/or heat strengthened). In certain example embodiments of this invention, the coated article includes a zinc stannate based layer provided over a silver-based infrared (IR) reflecting layer, where the zinc stannate based layer is preferably located between first and second silver based IR reflecting layers. In certain example embodiments, the zinc stannate based layer is provided between and contacting (i) an upper contact layer of or including Ni and/or Cr, and (ii) a layer of or including silicon nitride, so that for example the layer stack moving away from the glass substrate may include layers comprising the following materials: glass . . . Ag/NiCrOx/ZnSnO/SiN . . . Ag . . . Low-E coatings according to various embodiments of this invention may, for example, have two or three silver-based IR reflecting layers: It has surprisingly been found that the provision of the zinc stannate based layer results in a coated article having improved thermal stability upon heat treatment (HT). Such coated articles, if heat treated (e.g., thermally tempered), realize a low ΔE* value (glass side reflective and/or transmissive), such as a ΔE* value of no greater than 5.0, more preferably no greater than 4.0, within certain HT time periods such as one or more of 10 minutes, 16 minutes, and/or 24 minutes. Moreover, it has surprisingly been found that the provision of the zinc stannate based layer causes the product's glass side reflective and/or transmissive ΔE* value to be surprisingly reduced in a desirable manner upon HT compared to if the zinc stannate based layer was not present (e.g., compared to if the zinc stannate based layer was instead a tin oxide layer). Coated articles according to certain example embodiments of this invention may or may not be heat treated, and may be used in the context of windows such a monolithic or IG window units in example applications.
Accordingly, it would be desirable to provide a coated article that is characterized by one or more of: (i) desirable visible transmission, (ii) good durability, (iii) desirable coloration, (iv) desirable emissivity, (v) low haze, and/or (vi) thermal stability upon I-IT so as to realize a glass side reflective ΔE* value no greater than about 5.0, more preferably no greater than about 4.5, and most preferably no greater than about 4.0, within certain HT time periods such as one or more of 10 minutes, 16 minutes, and/or 24 minutes.
In certain example embodiments of this invention, there is provided a coated article including a coating supported by a glass substrate, comprising: a first dielectric layer supported by the glass substrate; a first infrared (IR) reflecting layer comprising silver supported by the glass substrate and located over at least the first dielectric layer; an upper contact layer comprising an oxide of Ni and/or Cr, the upper contact layer located over and directly contacting the first IR reflecting layer comprising silver; a layer comprising zinc stannate located over and directly contacting the upper contact layer comprising the oxide of Ni and/or Cr; a first layer comprising silicon nitride located over and directly contacting the layer comprising zinc stannate; a second IR reflecting layer comprising silver located over at least the first layer comprising silicon nitride; and another dielectric layer located over at least the second IR reflecting layer.
In certain example embodiments of this invention, there is provided a method of making a thermally tempered coated article, the method comprising: heat treating, at temperature(s) of at least 600 degrees C., a coated article including a coating supported by a glass substrate, the coating comprising a first dielectric layer supported by the glass substrate, a first infrared (IR) reflecting layer comprising silver supported by the glass substrate and located over at least the first dielectric layer, an upper contact layer comprising an oxide of Ni and/or Cr, the upper contact layer located over and directly contacting the first IR reflecting layer comprising silver, a layer comprising zinc stannate located over and directly contacting the upper contact layer comprising the oxide of Ni and/or Cr, a first layer comprising silicon nitride located over and directly contacting the layer comprising zinc stannate, a second IR reflecting layer comprising silver located over at least the first layer comprising silicon nitride, and another dielectric layer located over at least the second IR reflecting layer; and wherein (i) visible transmission of the coated article substantially plateaus and thus does not change by more than 1.0% between heat treating times of from 12-24 minutes during the heat treating, and/or (ii) the coated article has a haze % of no greater than 0.60% upon heat treatment for all heat treating time periods between 0 and 30 minutes.